It started at a bowling alley in Erlanger, Kentucky. A man she just met, a friend of a friend, bought her drink after drink. Later that night, she threw up, and passed out. Then, Valerie says, it happened.

"When I woke up the next morning, my panties and the sweatpants were down around my ankles and my bra was undone," she said.

Valerie said she realized she was raped. Reporting it the next day - a classic charge of acquaintance rape. Nearly three years later still no arrest in the case.

"I feel like, I almost fee like they're calling me a liar. That they don't believe me," Valerie said.

Rape in this country is surprisingly easy to get away with. The arrest rate last year was just 25 percent - a fraction of the rate for murder - 79 percent, and aggravated assault - 51 percent.

"When we have talked to victims, they very much so doubt that it was worth it for them to go to the police," said Sarah Tofte, US Program Researcher for Human Rights Watch. "They're incredibly disillusioned with the criminal justice system, and that sends a terrible message."

The suspect's attorney told police his client never had sex with Valerie. Yet an exam revealed "evidence of forced sexual penetration." Semen found on her underwear. Nurses took a rape kit- a collection of swabs and clothing that provide DNA evidence. The suspect provided a sample. But the DNA was never tested.

"Testing the kit is one way to affirm a victim's story," Tofte said, "and discredit the suspect's story."

A five month CBS News Investigation has found a staggering number of rape kits -- that could contain incriminating DNA evidence -- have never been sent to crime labs for testing.

At least 20,000 untested kits: 5,600 in Detroit. 3,800 in Houston. 5,100 in San Antonio, 1,100 in Albuquerque.

Many untested for years. And that's not all. At least twelve major American cities: Anchorage, Baltimore, Birmingham, Chicago, Cincinnati, Cleveland, Columbus, Indianapolis, Jacksonville, Oakland, Phoenix, San Diego said they have no idea how many of rape kits in storage are untested.

(FBI, Justice Department)

Police departments told us rape kits don't get tested due to cost - up to $1,500 a kit -- a decision not to prosecute, and victims who recant or are unwilling to move forward with a case.

Psychologist David Lisak from the University of Massachusetts has spent twenty years studying the minds of rapists.

"Somehow all we can do is take the statement from the victim. Take the statement from the alleged perpetrator and then throw up our hands because they are saying conflicting things," he said. "That's not how we investigate other crimes."

Valerie was told her rape kit wasn't tested because they didn't have the money. But when we caught up with Kenton County prosecutor, Rob Sanders, he told us something else.

"The results of the DNA test would not have made the case one way or another," Sanders said.

Sanders said his office made a "judgment call" the case was unwinnable in court -- claiming there were issues with Valerie's memory and the alcohol involved. A practice, says Lisak that often plays right into the hands of rapists.

"Predators look for vulnerable people and they prey on vulnerable people," Lisak said. And if, as a criminal justice system, we're going to essentially turn from any victim who was drinking or any victim who was in some way vulnerable - we're essentially giving a free pass to sexual predators."

Worried they were doing just that, CBS News has learned the Oakland California Police Department is now plowing through 489 untested rape kits from stranger rapes dating back six years, looking for evidence in what they believe to be "solvable cases."

The Los Angeles Police Department is testing a backlog of nearly 3,000 rape kits. LAPD's new Chief Charles Beck says efforts to reduce the backlog have "resulted in 405 hits" in the FBI DNA database.

In New York City, prosecutors are even more aggressive - testing every rape kit, even in cases of acquaintance rape - over 1,300 last year alone.

"You never know what you're going to find," said Mecki Prinz of the NY Medical Examiners Office.

The results are stunning. Today New York City's arrest rate for rape is 70 percent - triple the national average.

Prinz says testing kits in acquaintance cases can tie suspects to other attacks, "We have lots of situations where a domestic situation or an acquaintance situation is actually an indication of the male involved responsible for other rapes," she said.